This time of year with the flies and summer heat, flies will lay eggs on poop. The maggots hatch out, and if any chickens have soiled vents or a little injury or prolapse, it can cause fly strike. It is deadly. So inspect the vents. I have used permethrin spray inside my coop in hot weather to control flies for up to 30 days. Fans, screen wire, and keeping droppings stirred, dry, and picked up helps a lot to prevent this. Here is a good article:
https://the-chicken-chick.com/flystrike-in-backyard-chickens-causes/
https://the-chicken-chick.com/flystrike-in-backyard-chickens-causes/
